U.S. Pat. No. 4,793,703 (Fretz, Jr.), assigned to the assignee of the present application, discloses the fabrication of three-layer composite lenses especially suitable for use in ophthalmic applications. As described in that patent, the lenses were comprised of a thin layer of inorganic glass, desirably exhibiting photochromic behavior, which is bonded to an organic plastic layer by means of an adhesive. The fabrication of three- and five-layer lenses comprising glass and polymer layers adhesively bonded together was reviewed in the patent. The prior laminated structures differed from those disclosed in the patent in the following two vital points:
First, the previous workers used an organic polymer (CR-39) exhibiting a linear coefficient of thermal expansion greater than one order of magnitude higher than that of the glass element; 1200-1500.times.10.sup.-7 /.degree. C. over the range of 0.degree.-100.degree. C. compared to 60-120.times.10.sup.-7 /.degree. C. over the range of 0.degree.-300.degree. C. In response to that critical problem, the patent discloses the use of organic polymer elements demonstrating linear coefficients of thermal expansion over the range of 0.degree.-100.degree. C. of about 200-700.times.10.sup.-7 /.degree. C., preferably about 400-600.times.10.sup.-7 /.degree. C.
Second, the previous workers did not use an adhesive that exhibits flexibility and which can be cured at temperatures in the vicinity of room temperature.
Resins disclosed as being operable as the organic plastic elements were selected from the group of acrylics, imide-modified acrylics, epoxies, polycarbonates, silicones, urethanes, and polyimides, the requirement being that they manifested a linear coefficient of thermal expansion between about 200-700.times.10.sup.-7 /.degree. C. Epoxy resins comprised the preferred materials.
Resins disclosed as being operable as the organic polymer adhesive were selected from the group of acrylics, epoxies, polycarbonates, silicones, and urethanes, those resins being required to exhibit flexibility and the capability of being cured at temperatures in the vicinity of room temperature. Adhesives derived from epoxy resins comprised the preferred materials.
The preferred glass elements comprised photochromic glasses. A photochromic glass marketed by Corning Incorporated (formerly Corning Glass Works), Corning, NY under the trademark PHOTOGRAY EXTRA, the composition of which is included within U.S. Pat. No. 4,190,451 (Hares et al.), was provided as an illustration of such glasses.
Further development effort to improve upon the basic laminated structures described in U.S. Pat. No. 4,793,703 has led to a three-layer composite comprising an inorganic glass layer or cap consisting of a photochromic glass strengthened via an ion exchange reaction bonded to an organic element prepared from an epoxy resin through a thermoplastic polyurethane adhesive. To prepare the laminate lenses for ophthalmic applications, the epoxy element (which faces the eye of the wearer) is ground and polished to prescription and thereafter coated with a tintable hard coating. The lens is then edged to fit into a frame. The edges of most of the lenses will also be beveled to aid in securing the lens in metal or plastic frames. Some of the lenses will be tinted to meet customer fashion desires. It will be appreciated that, while in use, all of the lenses will be exposed to a variety of sources of moisture, such as perspiration, cleaning fluids, and atmospheric moisture. Edging, beveling, tinting, and exposure to moisture have led to three edge-related problems.
Many of the dyes used in tinting the organic polymer elements are water-based. Those dyes have been observed to be readily absorbed by the thermoplastic polyurethane adhesive. Such absorption results in a band of high coloration around the edge of the lens after fashion tinting. That band is cosmetically objectionable with rimless and clear plastic frames. Furthermore, over a period of several months the tinting migrates into the adhesive toward the center of the lens. That action creates a darkly colored halo in the lens which is again cosmetically unacceptable.
The strengthening of the glass cap via ion exchange must be undertaken prior to laminating the components of the lens together, inasmuch as the laminated composite will not withstand temperatures substantially above 130.degree. C. The edging process removes the ion exchanged layer at the edge of the glass element, thereby sharply decreasing the strength of the glass at the edge.
Beveling further reduces the strength of the edge glass. The bevel comprises a steepled ridge around the circumference of the lens. For both cosmetic and processing reasons, this ridge will preferentially be centered over the adhesive. Thus, centering the steepled portion in that manner hides the adhesive under the frame, a cosmetic advantage. During the beveling process, the adhesive, which doesn't grind well, moves away from the grinding surfaces to center on the groove in the wheel. That action imparts a knife edge to the glass element at the edge. The weakened thin glass edge is readily susceptible to chipping when the lens is inserted into the frame or when the frame is tightened down onto the bevel after insertion thereinto. Chips have also appeared after the eyewear has been in service for some time. The development of such chips may be due to the frames flexing against the weakened glass edge during normal use. It has also been posited that such delayed appearance of chips may be due to localized swelling of the polyurethane adhesive upon exposure to moisture which adds stresses to the weakened glass edge.
Some glass caps have evidenced a tendency to crack after a period of time when tested at high humidity and elevated temperatures. It appears that moisture must be present for the cracks to occur. The cracks originate at the edge and propagate toward the center of the cap. It is believed that moisture propagates tiny flaws that are generated during edging and beveling. It has also been surmised that the problem may be exacerbated by moisture slowly swelling the adhesive exposed at the edge which applies further stresses to the weakened glass edge.
Hence, as has been explained above, the fabrication of laminated lenses of the type disclosed in U.S. Pat. No. 4,793,703 has been hampered by three problems; viz., the absorption of tint by the adhesive, the glass chipping and/or cracking during framing, and delayed chipping of the glass in the presence of moisture. Solutions to the individual problems were investigated, those solutions adding processing steps. Therefore, the primary objective of the present invention was to devise a single means for solving all three of the cited problems.
Whereas the problems of glass chipping and/or cracking during framing and delayed chipping of the glass in the presence of moisture are particularly severe with laminated lens structures, the same problems are likewise present, but to a lesser degree, with single element glass lenses. Thus, single element glass lenses must also be edged and beveled prior to being secured into frames. And, therefore, the edges of those lenses are subject to the same stresses and exposure to moisture as are encountered by the laminated lenses. Accordingly, it was believed that an inventive solution for the problems faced by the laminated lens structures would be equally applicable to single element glass lenses.
The use of a single element plastic or glass lens quite apparently eliminates the need for the edge coating to contain a tint barrier inasmuch as there is no adhesive layer exposed to absorb tint. Furthermore, as has been explained above, the protection to chipping provided by the edge coating is not as critical as is required in laminated lens structures. There is one especially useful function, however, which the coating performs for both the laminated lens structures and the single element lenses; viz., the ability to recover lenses which have been sized too small for the frame. To explain:
Occasionally a lens will be cut slightly too small during processing to fit securely in the frame. On such occasions the lens must either be discarded and a new lens processed, or the lens or frame must be modified to obtain a sound fit. The most common method of accomplishing that modification is to use what has been termed a "lens liner". This lens liner material is supplied as a thin V-shaped ribbon which is inserted between the bevel of the lens and the frame to fill the space between the frame and undersized lens. The lens liner, however, is not cemented or otherwise attached to either the lens or the frame, and is difficult to hold during insertion of the lens into the frame. Moreover, because it is not attached to either the frame or the lens, loss of the lens by slipping out of the frame is hazarded. The inventive edge coating possesses three important advantages over the lens liner:
(1) because the edge coating becomes an integral part of the lens, insertion of the lens into the frame is easy; PA1 (2) because the edge coating is bonded to the lens, the potential for loss of the lens through slipping out of the frame is removed; and PA1 (3) because of the compliant nature of the coating, it readily and accurately conforms to any minor irregularities in the frame, thereby providing a better fit. PA1 (a) it must be soft enough to provide cushioning between the lens and the frame; PA1 (b) it must be capable of acting as a moisture barrier to inhibit penetration of water to the glass and the adhesive; and PA1 (c) it must be resistant to absorption of the tinting dyes to protect the adhesive from exposure to the dyes.